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Four Stroke SI Engine

Stroke 1: Fuel-air mixture introduced into cylinder

through intake valve

Stroke 2: Fuel-air mixture compressed

Stroke 3: Combustion (~constant volume) occurs

and product gases expand doing work

Stroke 4: Product gases pushed out of the cylinder

through the exhaust valve

Compression

Stroke

Power

StrokeExhaust

Stroke

AI

R

Combustion

Products

Ignition

Intake

Stroke

FUEL

Fuel/AirMixture

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Crank shaft

90o

180o

BC

TC

0o

270o

U

Spark plug for SI engine

Fuel injector for CI engine

Top

Center

(TC)

Bottom

Center

(BC)

Valves

Clearance

volume

Cylinder

wall

Piston

Stroke

Cylinder

Components

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IVO - intake valve opens, IVC intake valve closes

EVO exhaust valve opens, EVC exhaust valve opens

Xb burned gas mole fraction

Four-Stroke SI Engine

Exhaust gas

residual

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Two Stroke SI Engine

Intake (Scavenging)

CompressionIgnition

ExhaustExpansion

Fuel-air-oil

mixture

Fuel-air-oil

mixture

compressed

Crank

shaft

Checkvalve

Exhaust

port

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Cross Loop Uniflow

Two-Stroke Scavenging

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EPO exhaust port open

EPC exhaust port closed

IPO intake port open

IPC intake port closed

Exhaust area

Intake area

Two-Stroke SI Engine

scavenging

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Cylinder Arrangement

Single-cylinder engine gives one power stroke per crank revolution(2 stroke) or two revolutions (4 stroke). The torque pulses are widely

spaced, and engine vibration and smoothness are significant problems.

Used in small engine applications where engine size is more important

Multi-cylinder engines spread out the displacement volume amongstmultiple smaller cylinders. Increased frequency of power strokes

produces smoother torque characteristics. Engine balance (inertia forces

associated with accelerating and decelerating piston) better than single

cylinder.

Most common cylinder arrangements:- In-line 4-cylinder

- In-line 6-cylinder

- V-6 and V-8

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Power Regulation (Throttling)

An IC engine is basically an air engine, the more air you get into the

cylinder, the more fuel you can burn, the more power you get out.The initial pressure in the cylinder is roughly equal to the pressure

in the intake manifold.

Pressure in the intake manifold is varied by opening and closing the

throttle plate to change the pressure drop. Maximum air flow (andpower) achieved at wide-open-throttle (WOT). Minimum air flow

at idle

Patm Pint < Patm

Intake manifold

Fuel

WOT

Idle

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Basic Carburetor Design

Venturi

Throttle

Air Flow

Mixture to manifold

Fuel

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Fuel Injection System

Throttle

Fuel tank

Air intakemanifold

During start-up the components are cold so fuel evaporation is very slow, as a result

additional fuel is added through a second injecting valve

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Superchargers and Turbochargers increase the power of an IC engine

by raising the intake pressure and thus allowing more fuel to be burned

per cycle. Knock or autoignition phenomenon limit precompression.

Superchargers are compressors that are mechanically driven by the

engine crankshaft and thus represent a parasitic load.

Compressor

PatmPint > Patm

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Turbochargers couple a compressor with a turbine driven by the exhaust

gas. The compressor pressure is proportional to the engine speed

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The peak pressure in the exhaust system is only slightly greater than

atmospheric small (P across turbine.

In order to produce enough power to run compressor the turbine speedmust be very fast (100k-200k rev/min) long term reliability an issue.

It takes time for turbine to get up to speed so when the throttle is opened

suddenly there is a delay in achieving peak power - Turbo lag.

Waste gate valve controls the exhaust gas flow rate to the turbine.

It is controlled by the intake manifold pressure

INTAKE

AIR

EXHAUSTFLOW